Continuity mistake: When Shane tackles Jim to the ground, Jim tells the group about how the walkers got his family. Jim isn't wearing his cap on his head, but as the shot is about to cut to the next one, Jim is wearing his cap.
Continuity mistake: As the scene opens on the rooftop with Daryl, T-Dog, Rick, and Glenn observing the area that Merle had been handcuffed to the pipe, blood splatter magically appears around the severed hand when it wasn't there at the end of the previous episode. (00:05:10)
Answer: The walkers could have knocked them down more than once. There may have been another wave that knocked down the fence earlier and they're going to put it up again soon.
Captain Defenestrator